This disclosure relates to the use of optical lidar in measuring the rotational velocity of an object.
Present technology in measuring the rotational velocity of an object often requires contact with the surface of the object, or is restricted in the size of the rotating plane, measurement geometry or linearity. Applications for coherent Doppler lidars include velocity sensing applications (platforms and objects), volumetric/fluidic flow sensing, vibration monitoring, range to target and other related standoff sensing applications such as rotational velocity. A Doppler lidar detects the Doppler frequency shift imposed on coherent light scattered from a moving target by mixing the scattered (or reflected), frequency shifted light with a reference beam of light (local oscillator) which is not shifted in frequency on the detector. As in the mixer of a conventional radio set, a difference frequency results from this mixing process which is proportional to the velocity of the scattering target. It is the Doppler frequency shift imposed on the light scattered from the target that provides the mechanism used for velocity detection. The reference beam can be either derived from the transmit beam (homodyne operation) or derived from another stable coherent source (heterodyne operation). By measuring the Doppler shift from three (or more) angularly separated lidar beams brought to a common focus point on an unconstrained, rotating object, a complete vector velocity can be computed from the center frequencies of the Doppler spectrums obtained, along with statistical velocity information. The optical assembly required to do this however is complicated.
The disclosed invention concept utilizes a homodyne/heterodyne interferometer technique in a modified lidar in such a manner as to sense the rotational velocity of a rotating (or xe2x80x9cspinningxe2x80x9d) object in either the Doppler bandwidth of a single axis system or in the differential spectrum of a xe2x80x9ctwoxe2x80x9d axis system. The technique as disclosed is based on optical fiber lidar techniques, but can be implemented in free-space optics as well. The disclosed invention therefore comprises both a technique for utilization The disclosed technique can be implemented for instance to measure the rotational velocity of a high velocity projectile in free space or a miniature shaft. Resolution is limited by the bandwidth of the lidar source and the focusing ability of the optical aperture. The disclosed technique allows for optical isolation of the sensor from the target surface and environment. Extremely high rotational velocities may be sensed with the disclosed technique.
The disclosed invention senses the roatational velocity of a rotating objected constrained to a single axis of rotation. In this case, with the appropriate measurement geometry, both the Doppler center frequency and bandwidth are proportional to velocity. As will be shown, the center frequency depends on the radial position of the detection beam(s). The bandwidth of the Doppler spectrum does not. Hence the Doppler bandwidth may be processed to implement a sensor which can interrogate a rotating surface at a substantial standoff distance, whereby the interrogation site is rotationally and positionally invariant. Properly arranged, the disclosed system can determine translational velocity and rotational velocity simultaneously. Velocities well into the hypersonic range are detectable with the disclosed concepts.